The present invention generally relates to devices for dissipating thermal energy from electronic components, and more particularly to a heat sink having the capacity to control differential thermal expansion in an electronic component system.
Modern electronic systems frequently use high-power hybrid circuitry containing numerous heat-generating components. For example, driver circuits for video display devices often contain substantial numbers of resistors, integrated circuits, and other semi-conductor components. These components generate varying amounts of heat during operation. In addition, heat load shifts occur from component to component, depending on the images displayed by the cathode ray tube of the device.
To dissipate heat from the above-described components, heat sinks are attached to the substrate on which the components are mounted. A heat sink is a device placed in physical contact with a heat-generating component to absorb and dissipate heat from the component.
Conventional heat sinks used in high-power hybrid circuitry are constructed of aluminum. Aluminum has a high thermal conductivity, which is desirable for a heat sink. Thermal conductivity is defined as the amount of heat per unit time that passes through a unit volume per degree of driving force. Thermal conductivity, as used herein, refers to values at the ordinary operating temperatures of the materials involved, typically between 20.degree.-150.degree. C. The thermal conductivity of aluminum within that temperature range is about 0.53 cal/cm-sec-.degree. C.
As previously described, heat sinks are conventionally secured to the substrate on which the components are mounted. However, in high-power circuitry having multiple heat-generating sources, the use of a heat sink having a coefficient of thermal expansion (CTE) not matched with that of the substrate causes frequent problems. For a given material, CTE is a numerical value involving the change in size per degree change in temperature. CTE values for solid materials vary with temperature in a non-linear manner. They are traditionally expressed as a linear average of the CTE v. temperature curve for the temperature range in which the product is most likely to be used.
If the CTE of the heat sink and substrate are not properly matched, heat produced by the heat-generating components in the circuit will cause the heat sink and substrate to expand differentially. As a result, significant mechanical stress is generated, causing performance and reliability problems including micro-cracking of the substrate and/or components, delamination of the components, and other stress-related problems. These difficulties have been observed in systems using aluminum heat sinks and substrates manufactured of beryllia (BeO) or alumina (Al.sub.2 O.sub.3). Aluminum has a relatively high CTE in comparison with BeO and Al.sub.2 O.sub.3, as shown in Table I.
TABLE I ______________________________________ Material CTE (ppm/.degree.C.) ______________________________________ Aluminum 23.0 (20-100.degree. C.) Beryllia 7.5 (25-400.degree. C.) Alumina (96%) 7.2 (25-400.degree. C.) ______________________________________
A need therefore exists for a heat dissipation device having a high thermal conductivity and a CTE substantially matched with the substrate to which it is secured. The present invention satisfies this need as described herein.